Quadrupole focusing of electron beams

ABSTRACT

A FOCUS COIL ASSEMBLY COMPRISES FOUR INTERWINED HELICES SURROUNDING AN ELECTRON BEAM TO-BE-FOCUSED. OPPOSITELY DIRECTED CURRENTS IN ADJACENT HELICES PRODUCE A CONTINUOUS AND DISTRIBUTED QUADRUPOLE MAGNETIC FOCUSING FIELD. FIELD CONCENTRATION CAN BE ACHIEVED BY MEANS OF A MAGNETIC YOKE SURROUNDING THE HELIX ASSEMBLY.

Feb. 9, 1971 c. M. DE SANTIS ETAL 3,562,683

QUADRUPOLE FOCUSING OF ELECTRON BEAMS Filed April 7, 1969 2 Sheets-Sheetl "\.S n I "2 5 Q g; m

INVENTORS.

i wam M 5M ATTORNEYS Feb. 9, 1971 c. M. DE SANTIS ETAL 3,562,683

QUADRUPOLE FOCUSING OF ELECTRON BEAMS Filed April 7, 1969 2 Sheets-Sheet2 t m 'INVENTORS.

. \v CHARLES M. DE SANTIS BRUNO ZOTTER BY r-I 53 m Jud-42 M r w rmm3,562,683 QUADRUPOLE FOCUSING OF ELECTRON BEAMS Charles M. De Santis,Neptune, N,J., and Bruno Zotter, Geneva, Switzerland, assignors to theUnited States of America as represented by the Secretary of the ArmyFiled Apr. 7, 1969, Ser. No. 814,023 Int. Cl. H01f /00 US. Cl. 335-213 3Claims ABSTRACT OF THE DISCLOSURE A focus coil assembly compirses fourintertwined helices surrounding an electron beam to-be-focused.Oppositely directed currents in adjacent helices produce a continuousand distributed quadrupole magnetic focusing field. Field concentrationcan be achieved by means of a magnetic yoke surrounding the helixassembly.

The invention described herein may be manufactured, used and licensed byor for the Government for govern mental purposes without the payment tous of any royalty thereon.

This invention relates to a magnetic focusing means for electron beamsand more particularly to a novel and useful quadrupole focusing meanscomprising a number of current carrying wires disposed around and alongan electron beam so that continuous or distributed quadrupole focusingof the beam is achieved. Quadrupole focusing has been used in the pastin particle accelerators and other high energy devices by utilizing apair of lumped focusing magnets or windings located at discrete pointsalong the beam. Since each such lumped quadrupole focusing means focusesthe electrons in only a single direction, at least a pair of such meansis required, with their focal axes displaced by 90 degrees, thusproviding omnidirectional focusing. These prior art lumped focusingcoils or magnets have not been extensively used because of the largediameter required of the focusing windings, pole pieces, and yokes. Thepresent invention comprises a single or unitary focusing coil as semblyadapted to surround an electron beam to provide a continuous field whichfield rotates in direction as it progresses along the electron beam, sothat a continuous quadrupole field results. Thus the coil is distributedover a large area and the outside dimension thereof is reduced, and alsothe resulting continuous focusing is more effective than the lumpedfocusing of the prior art. The distributed field may be provided bymeans of four helically twisted and intertwined currentcarrying wiresarranged 90 degrees apart around the electron beam to-be-focused. Directcurrent is sent through adjacent coils in opposite directions toestablish the required quadrupole field.

It is thus an object of the invention to provide a continuous ordistributed quadrupole focusing means.

A further object is to provide a quadrupole magnetic electron beamfocusing means of small outside dimensions.

Another object is to provide an economical and compact unitary magneticfocusing coil for use with cathode ray tubes.

These and other objects and advantages of the invention will becomeapparent from the following detailed description and drawings, in which:

FIG. 1 is a pictorial view of a pair of prior art focusing means.

FIGS. 2 and 3 are schematic diagrams of focusing coils constructedaccording to the present invention.

FIG. 1 shows a pair of lumped prior art quadrupole focusing coils withtheir yokes. In the end view of FIG.

United States Patent 0 la it is seen that each such assembly comprises aringshaped yoke 5 with four internally-projecting pole pieces spaced atdegree intervals thereon. Each pole piece has a winding 7 thereon andthe current in each of the four windings is in such a direction thatdiametrically opposite pole pieces are of the same magnetic polarity, asshown by the letter N and S thereon. FIG. 1b shows a side view of a pairof these focus coils disposed along an electron beam, the direction ofwhich is indicated by the dashed line 9. As mentioned above, two suchlumped focus coils are required with a 90 degree spacing between themagnetic axes thereof. Thus the left hand coil, 5,. of FIG. 1b has itstwo north poles in the vertical plane and the right hand coil, 5', hasits south poles in the same plane, so that the magnetic field patternwill have the required twist. The field line patterns within each focuscoil are shown in FIG. 1a by the dashed lines, the flux directionfollowing the north to south pole convention.

FIG. 2 is a schematic diagram showing how four intertwined helices canbe connected to provide a continuous, distributed quadrupole focusingcoil of unitary structure. FIG. 2b is a side view and the two end viewsof FIGS. 2a and 20 show how the helix ends are electrically connected toachieve the desired quadrupole field pattern. The opposite ends of eachof the one-turn helices are labelled 1-1, 2-2, 3-3 and 4-4. The solidlines in FIG. 2b indicate that this part of the helix is in front of thehelix axis 9 and the dashed line portions are to the rear of the axis.Thus the first helix starts at point 1 directly above the helix axis 9and progresses after 1 turn to the same relative position, 1, at theright hand end. The second helix, 2-2, begins 90 degrees away from thefirst, which puts point 2 and 2 in front of the axis 9 and toward theviewer in FIG. 2b. The third helix 3-3 begins and ends directly belowthe axis 9. The fourth helix 4-4 is not separately visible in FIG. 2bsince it is directly behind the second helix 2-2, and therefore degreesdisplaced therefrom. The arrowheads on FIG. 2b indicates the directionsof the direct currents required in each helix to achieve the quadrupoletype magnetic field within the helix. It can be seen that the currentdirection in each helix is opposite to that of both adjacent helices or,to state the same thing in other words, current direction in each helixalternates as one progresses around the coil periphery. FIGS. 2a and 20show how a single D.C. power supply can be used to properly energize allof the helices if the illustrated jumpers are used at either end of thehelix assembly. One power supply terminal, shown as positive, isconnected to point 1, points 1 and 2, 2 and 3, and 3 and 4 are connectedby jumpers, and the negative or ground power supply terminal connectedto point 4. Each helix is shown as comprising one turn for simplicity ofillustration, however, in practice other numbers of turns may beutilized.

FIG. 3a is a side view of a focus coil assembly with a pair of terminals13 and 15 corresponding to points 1 and 4 of FIG. 2. In order to providemagnetic and electric shielding as well as to provide an ironcladmagnetic circuit with its attendant advantages, the helix assembly issurrounded with a stack of disc-shaped yokes 12. FIG. 3b shows an endview of one of these discs. It is similar in shape to the yokes 5 and 5of FIG. 1. The four recesses 17 in the disc can be designed toaccommodate the four helical windings, so that each disc can be screwedor threaded onto the completed rigid helical assembly, which would forma screw thread for each disc. The resulting projections 19 would thenform pole pieces which would follow the helical shape of the fourhelices. If the pole pieces 19 of the yokes 12 are made from permanentmagnets with adjacent pole pieces of opposite magnetic polarity, thehelices and the power supply can be eliminated, if the discs 12 arestacked in a helical fashion. An alternate construction, notillustrated, would be to wind magnetic wire around the helix assembly,so that the magnetic wire would fill in all the spaces between theadjacent helices. It would be necessary to insulate the magnetic wirefrom the helices. Insulation would also be required in the embodiment ofFIG. 3 between the magnetic discs and the helix assembly.

Thus it can be seen that the present invention achieves quadrupolefocusing with a simpler, less bulky and more compact structure than hadbeen available heretofore. The helical structure may advantageouslyserve a dual purpose as part of a slow wave structure, for instance in atraveling wave tube. The entire focusing structure may also be locatedwithin the evacuated envelope of a cathode ray tube.

It should be understood that we do not desire to be limited to the exactdetails of construction shown and described, since obvious modificationswill occur to those persons skilled in this art.

What is claimed is:

1. An electron beam focusing coil assembly comprising, ct'ourintertwined current-carrying helices disposed around and along saidelectron beam, corresponding points of adjacent helices being displacedby 90 degrees,

and means to establish direct current in opposite directions in adjacentones of said helices.

2. The coil assembly of claim 1 wherein a magnetic yoke surrounds saidhelices, said yoke comprising a stack of magnetic discs adapted to bethreaded over said helices, and insulated therefrom.

3. An electron beam focusing coil assembly comprising, four intertwinedcurrent-carrying helices disposed around and along said electron beam at90 degree intervals, means to establish direct current in oppositedirections in adjacent ones of said helices, and wherein a magnetic yokesurrounds said coil assembly, said yoke comprising a stack of magneticdiscs adapted to be threaded over said helices.

References Cited UNITED STATES PATENTS 2,155,514 4/1939 Tolson et al.313-76 2,243,893 6/ 194-1 Blumlein 313-76 3,284,744 11/1966 Dandy et a1.31384X GEORGE HARRIS, Primary Examiner US. Cl. X.R. 315-5.35; 31384

